The sclera is the white, opaque outer layer of the eye that provides structural support and protection. Composed mainly of collagen and elastic fibers, the sclera is tough and durable, forming the majority of the outer coating of the eye. It serves as the attachment point for the extraocular muscles that control eye movement and helps maintain the shape of the eye. The sclera also ensures that light does not pass through the eye except through the cornea.
Location
The sclera surrounds most of the eye, extending from the cornea at the front of the eye to the optic nerve at the back. It is continuous with the cornea at the limbus and covers the entire posterior segment of the eye, except for a small opening at the optic nerve where it allows the nerve fibers to exit the eye and connect to the brain.
Structure and Anatomy
The sclera is a dense, fibrous structure that forms the outer protective layer of the eye. Its anatomy is designed to provide structural integrity and protection to the internal components of the eye. Below is a detailed description of the sclera’s anatomy.
Location and General Structure
- The sclera constitutes about 80% of the outer coat of the eye, forming a continuous shell that surrounds the eye except for the transparent cornea in the anterior region.
- It begins at the limbus, where it meets the cornea at the front of the eye, and extends to the optic nerve at the back, where it has an opening for the optic nerve fibers to pass through.
- The sclera is opaque, providing the white appearance of the eye, and it is much thicker in the posterior region than in the anterior region.
Layers of the Sclera
The sclera is composed of three distinct layers, each contributing to its strength and resilience:
Episclera
- The episclera is the outermost layer of the sclera, lying just beneath the conjunctiva (the mucous membrane that covers the eye and inner eyelids).
- It is made up of loose connective tissue with collagen fibers and is richly supplied with blood vessels.
- The episclera connects to the Tenon’s capsule, a thin layer of connective tissue that facilitates smooth movement of the eye within the orbit.
Scleral Stroma (Substantia Propria)
- The scleral stroma is the thickest and most substantial layer of the sclera. It is composed mainly of dense irregular connective tissue, with large amounts of collagen and some elastic fibers.
- The arrangement of collagen fibers in the stroma is less regular than that in the cornea, which accounts for the sclera’s opacity.
- The stroma provides the bulk of the sclera’s strength, helping maintain the shape of the eyeball and providing protection against external forces.
Lamina Fusca
- The lamina fusca is the innermost layer of the sclera, located adjacent to the choroid.
- This layer is thinner and contains pigmented cells (melanocytes), which help in preventing light from passing through the sclera and scattering within the eye.
- The lamina fusca also contains elastic fibers that connect the sclera to the underlying choroid, allowing a degree of flexibility between these layers.
Thickness
- The sclera varies in thickness depending on its location:
- Anterior Sclera: In the front of the eye, near the limbus, the sclera is relatively thin, measuring about 0.3 to 0.4 mm in thickness.
- Equator of the Eye: Around the midsection or equator of the eyeball, the sclera increases in thickness to about 0.6 mm.
- Posterior Sclera: At the back of the eye, near the optic nerve, the sclera is thickest, measuring up to 1.0 mm.
- The variation in thickness contributes to the overall strength and flexibility of the sclera, allowing it to withstand internal and external pressure.
Openings in the Sclera
The sclera has several openings to accommodate various structures, including nerves and blood vessels:
Anterior Openings
The anterior openings are for the passage of the anterior ciliary arteries and veins, which supply blood to the anterior segment of the eye, including the iris and conjunctiva.
Middle Openings
The middle openings are located around the equator of the eye and are for the passage of the vortex veins, which drain blood from the choroid and retina.
Posterior Openings
- The largest and most important opening is the scleral canal, located at the back of the sclera. This opening allows the optic nerve fibers to exit the eye and travel to the brain.
- Around the optic nerve canal are smaller openings for the passage of the short and long posterior ciliary arteries and nerves, which supply the choroid and optic nerve head.
Scleral Spur
- The scleral spur is a circular ridge formed by the thickened part of the sclera near the limbus, where it meets the cornea.
- This structure is significant because it serves as the attachment point for the ciliary muscle and the trabecular meshwork. The ciliary muscle is responsible for controlling accommodation, while the trabecular meshwork helps drain aqueous humor from the anterior chamber of the eye.
Blood Supply
- The sclera is largely avascular, meaning it has no direct blood supply in most areas. However, it receives nourishment through the surrounding tissues:
- Anterior Sclera: Supplied by branches of the anterior ciliary arteries.
- Posterior Sclera: Supplied by the short and long posterior ciliary arteries, which also provide nutrients to the adjacent choroid.
- Episclera: Rich in blood vessels, providing some nourishment to the outer layers of the sclera.
Nerve Supply
- The sclera is innervated by branches of the ophthalmic division of the trigeminal nerve (cranial nerve V), primarily through the long ciliary nerves and short ciliary nerves.
- These nerves provide sensory innervation to the sclera, allowing it to detect pain, pressure, and other sensations.
Relationship with Surrounding Structures
- Cornea: The sclera is continuous with the cornea at the limbus, where the transparent cornea merges with the opaque sclera. The transition from the clear cornea to the white sclera marks the junction between the anterior transparent region and the outer protective layer.
- Choroid: The inner surface of the sclera is in contact with the choroid, the vascular layer that nourishes the retina. The two layers are connected via the lamina fusca.
- Optic Nerve: The sclera encircles the optic nerve at the back of the eye, providing protection to the nerve fibers as they exit the eye through the lamina cribrosa, a specialized region of the sclera with small perforations for the nerve fibers to pass through.
Elasticity and Flexibility
- The sclera has a degree of elasticity and flexibility, allowing it to adjust to changes in intraocular pressure (IOP) without rupturing.
- This flexibility is especially important for protecting the internal structures of the eye, such as the retina and choroid, from damage caused by sudden impacts or pressure changes.
Function
The sclera, commonly known as the “white of the eye,” plays several crucial roles in maintaining the structure, protection, and function of the eye. Below is a detailed explanation of the primary functions of the sclera.
Structural Support and Maintenance of Eye Shape
The sclera provides structural integrity to the eye and helps maintain its shape:
- Protective Shell: The sclera forms a tough, fibrous outer shell that encloses and supports the internal structures of the eye, including the retina, choroid, and vitreous body. This rigid framework helps protect the delicate tissues inside the eye from external damage.
- Maintaining Eye Shape: The spherical shape of the eye is maintained by the sclera, which resists deformation caused by internal pressure and external forces. This shape is critical for proper light refraction and the focusing ability of the eye.
- Resistance to Internal Pressure: The sclera provides resistance to changes in intraocular pressure (IOP), which is essential for keeping the eyeball from collapsing or expanding due to fluctuations in internal fluid pressure.
Protection of Internal Eye Structures
The sclera acts as a protective barrier that shields the inner eye from injury, infection, and other potential harm:
- Mechanical Protection: The sclera’s tough collagen structure is designed to resist physical impacts and trauma, safeguarding the sensitive tissues inside the eye, such as the retina, lens, and vitreous body, from damage.
- Barrier Against Infection: The sclera forms a barrier to pathogens, helping to protect the internal structures of the eye from bacterial, viral, and fungal infections. Although the sclera itself is avascular, the outer episclera contains blood vessels that can mount an immune response if necessary.
- Prevention of Light Penetration: The sclera is opaque, which prevents light from entering the eye except through the cornea. This helps ensure that only properly focused light enters the eye, reducing the risk of blurred or distorted vision due to scattered light entering from other areas.
Anchor for Extraocular Muscles
The sclera serves as the attachment site for the extraocular muscles, which control the movement of the eye:
- Extraocular Muscles: Six extraocular muscles—four rectus muscles (superior, inferior, medial, and lateral) and two oblique muscles (superior and inferior)—attach to the sclera. These muscles are responsible for moving the eye in various directions, allowing for coordinated eye movement, tracking, and focusing.
- Eye Movement: The sclera provides a stable and rigid surface for these muscles to exert force, ensuring precise and smooth movements of the eyeball. This coordination is crucial for maintaining proper alignment between the eyes and enabling binocular vision (depth perception).
Regulation of Intraocular Pressure (IOP)
The sclera indirectly contributes to the regulation of intraocular pressure (IOP), which is essential for the health and function of the eye:
- Resistance to Pressure Changes: The sclera provides mechanical support that helps resist changes in IOP, maintaining the overall shape and size of the eye. This prevents the eye from collapsing under low pressure or expanding under high pressure.
- Aqueous Humor Outflow: At the junction between the cornea and sclera (at the scleral spur), the trabecular meshwork and Schlemm’s canal are located, which are responsible for draining the aqueous humor from the anterior chamber. Proper scleral rigidity is essential for maintaining the outflow pathway and regulating IOP.
Facilitating Blood and Nerve Passage
The sclera contains openings for blood vessels and nerves that supply the inner structures of the eye:
- Optic Nerve Passage: The sclera forms a protective sheath around the optic nerve as it exits the eye. The opening for the optic nerve, known as the lamina cribrosa, allows nerve fibers to pass from the retina to the brain, where visual information is processed.
- Blood Vessel Openings: The sclera has openings for the ciliary arteries and vortex veins, which supply blood to the choroid and retina. These openings allow for essential nutrient exchange and waste removal within the eye, while the sclera provides a protective covering for these vessels.
Flexibility and Elasticity
The sclera provides a balance of strength and flexibility, enabling it to respond to external forces and maintain eye function:
- Elasticity: The sclera has elastic properties that allow it to adjust to slight changes in shape during movements or fluctuations in intraocular pressure. This flexibility is essential for maintaining the structural integrity of the eye while accommodating small deformations.
- Eye Movement Adaptation: During rapid eye movements or shifts in gaze, the sclera’s flexibility ensures that it can adapt to the mechanical forces exerted by the extraocular muscles without compromising the overall shape of the eyeball.
Transition to the Cornea and Limbus
The sclera forms a continuous outer layer of the eye, and it transitions smoothly into the cornea at the limbus, which plays several important roles:
- Supporting Corneal Transparency: The junction between the sclera and cornea helps maintain the transparency of the cornea by keeping the boundary free of vascular invasion. Blood vessels from the sclera do not penetrate the cornea, which is essential for the cornea’s transparency and refractive function.
- Providing Structural Support to the Limbus: The limbus contains stem cells that regenerate the corneal epithelium, and the sclera provides structural support to this region, ensuring the long-term health and transparency of the cornea.
Maintaining Ocular Size and Volume
The sclera helps maintain the overall size and volume of the eye:
- Preventing Eye Distortion: The sclera’s strong collagen structure prevents the eye from becoming misshapen due to changes in intraocular pressure or external trauma. By maintaining the spherical shape of the eye, the sclera ensures that the optical components (cornea and lens) remain properly aligned for effective light refraction.
- Support for Internal Structures: The sclera provides a solid framework that supports the delicate internal structures of the eye, including the retina and vitreous body, helping to preserve their proper positioning and function.
Clinical Significance
The sclera plays a crucial role in maintaining the shape, structure, and function of the eye, making it clinically significant in various eye conditions. Scleritis, an inflammatory condition of the sclera, can cause severe pain, redness, and swelling, often linked to autoimmune diseases like rheumatoid arthritis. If untreated, scleritis can lead to complications such as vision loss or damage to surrounding eye structures.
Episcleritis, a less severe form of inflammation, affects the outer layer of the sclera and usually resolves without long-term damage. Scleral thinning or degeneration, often seen in conditions like high myopia (nearsightedness) or after surgery, can weaken the eye’s structural integrity and lead to vision complications. Trauma or injuries to the sclera may result in scleral rupture, which is a medical emergency requiring immediate attention to prevent vision loss. Maintaining the health of the sclera is essential for the overall function and protection of the eye.
